1. Field of the Invention
The present invention relates to a thin film magnetic recording medium, which is composed of an underlayer film of a nonmagnetic metal that is formed on a nonmagnetic substrate by a sputtering method and a magnetic film that is formed on the underlayer film by an oblique evaporation process.
2. Description of the Related Art
Recently, recording density of a magnetic recording medium is rapidly advanced towards higher density. During the process of advancing towards higher density, it is commonly known that a magnetic recording medium has been shifted from an iron oxide tape having high coercive force and high magnetic flux density to a metal tape and a thin film magnetic recording medium (thin film magnetic tape) having higher performance.
With respect to an application of such a magnetic recording medium, in the field of a VTR (Video Tape Recorder), for example, a thin film magnetic tape has been gained attention so as to accomplish digitalization and high definition.
A so-called evaporation tape in which a magnetic film is formed by the oblique evaporation process has been put into practical use as a thin film magnetic tape.
By using a piercing electron gun that will be detailed later, such an evaporation tape is manufactured by such that an electron beam emitted form the piercing electron gun is applied to a magnetic material such as Co (cobalt) and CoNi placed in a crucible in a vacuum chamber, and the magnetic material is melted and evaporated while injecting oxygen gas, and then a thin film containing CoO and CoNiO is formed on a base film made of PET (polyethylene terephthalate), PEN (polyether naphthalate), PI (polyimide) or PA (polyamide).
FIG. 15 is a plan view of a general film-forming apparatus, which applies the oblique evaporation process, for producing a thin film magnetic recording medium according to the prior art. As shown in FIG. 15, a film-forming apparatus 20 for producing a thin film magnetic recording medium (thin film magnetic tape) maintains a vacuum condition inside a vacuum chamber 1 by a vacuum pump (not shown). Inside the vacuum chamber 1, there provided one set of film winding rolls 2 and 3, one set of tape guide rolls 4 and 5, and a cooling can roll 7.
During ordinarily forming a film on a base film 6, the base film 6 wound around the film winding roll 2 (hereinafter referred to as supply roll 2) runs through the tape guide roll 4, the cooling can roll 7, and the other tape guide roll 5 in a forward direction shown by arrows to the other film winding roll 3 (hereinafter referred to as take-up roll 3).
The base film 6 is made of, for example, a PET (polyethylene terephthalate) film having a predetermined thickness as a substrate for a thin film magnetic tape.
A cooling device (not shown) is installed inside the cooling can roll 5 so as to prevent deformation of the base film 6 caused by increased temperature during an evaporation process.
A crucible 8, which is formed in a box shape and contains a magnetic metal material 11 such as Co, is installed at a lower right hand corner from the cooling can roll 7 inside the vacuum chamber 1.
A piercing electron gun 12, which is an evaporation heat source to melt and evaporate the magnetic metal material 11 in the crucible 8, is mounted on a right wall 1a of the vacuum chamber 1 with pointing at the crucible 8 located diagonally downward to the left. The piercing electron gun 8 emits an electron beam 13 towards the magnetic metal material 11 inside the crucible 8. The electron beam 13 melts the magnetic metal material 11 and evaporates so as to coat a surface of the base film 6, which is moving along the cooling can roll 7.
It is essential to cover both edges of the base film 6 so as to prevent a magnetic metal vapor 11a, which evaporated from the crucible 8, from evaporating on the cooling can roll 7 while the base film 6 is running.
Further, it is also essential to control an incidence angle of evaporation of the magnetic metal vapor 11a such as evaporated Co with respect to a surface of the base film 6 due to the improvement of recording characteristics when producing a thin film magnetic tape. In order to prevent deposits in inappropriate areas, incidence angle controlling masks 9 and 10 are installed between the cooling can roll 7 and the crucible 8 as shown in FIG. 15.
Allocating one incidence angle controlling mask 9 to a predetermined position of the cooling can roll 7 controls a maximum incidence angle “θ max” of the magnetic metal vapor 11a of the magnetic metal material 11 with respect to the base film 6. On the other hand, allocating the other incidence angle controlling mask 10 to another predetermined position of the cooling can roll 7 controls a minimum incidence angle “θ min”.
While the base film 6 is running along the outer circumference of the cooling can roll 7, the magnetic metal vapor 11a is deposited on the surface of the base film 6 within a range of angle from a maximum incidence angle to a minimum incidence angle that is adjusted to a predetermined angle, wherein the range of angle is referred to as an evaporation opening area, and then a magnetic film is formed on the surface of the base film 6. Magnetic characteristics of the magnetic film are decided by the maximum incidence angle “θ max” and the minimum incidence angle “θ min” of the magnetic metal vapor 11a of the magnetic metal material 11. Generally, the maximum incidence angle “θ max” is set to 90 degrees and the minimum incidence angle “θ min” is set to 40 degrees.
An oxygen gas injection pipe (not shown) is installed between the cooling can roll 7 and the incidence angle controlling mask 10 inside the minimum incidence angle “θ min” side. Oxygen gas O2 blows off through a plurality of holes (not shown) provided on the oxygen gas injection pipe towards the magnetic metal vapor 11a evaporated from the crucible 8.
The electron beam 13 emitted from the piercing electron gun 12 is controlled by a deflection magnet 15, which supplies a deflection magnetic field onto a trajectory of the electron beam 13, and another deflection magnet 14, which is installed near the crucible 8. By scanning the electron beam 13 in the longitudinal direction of the crucible 8, the magnetic metal vapor 11a such as evaporated Co is thinly deposited on the surface of the base film 6 as a partial oxide magnetic film such as Co—CoO in a lateral direction of the base film 6. By depositing the partial oxide magnetic film on the base film 6 continuously in the longitudinal direction of the base film 6, a long enough thin film magnetic tape is taken up by the take-up roll 3.
There provided another scanning method of the electron beam 13 such that an ongoing-straight electron beam 13 emitted from the piercing electron gun 12 is controlled to scan the crucible 8 by only the deflection magnet 15 without installing the deflection magnet 14 near the crucible 8. Such a scanning method is also applicable.
FIG. 16 is a partially enlarged cross sectional view of a thin film magnetic recording medium (thin film magnetic tape) produced by the film-forming apparatus 20 shown in FIG. 15, which applies the oblique evaporation process, and exemplarily exhibits states of a nonmagnetic underlayer film and a magnetic film that is formed on the nonmagnetic underlayer film, which constitute a part of the thin film magnetic tape. In other words, FIG. 16 shows an exemplary configuration of the cross section of the thin film magnetic tape of which the magnetic film is cut along the longitudinal direction of the base film and is illustrated exemplarily from a cross sectional picture that is enlarged as large as 500 to 2500 thousands times, which is obtained by observing the cross section through the transmission electron microscope (model H-800 manufactured by Hitachi, Ltd.) at the acceleration voltage of 200 kV.
In FIG. 16, a reference sign 16 is a nonmagnetic underlayer film that is composed of a nonmagnetic metal oxide film such as CoO having a columnar structure, which is formed on a nonmagnetic substrate to be a base film (not shown) by controlling an injection amount of oxygen gas and non-magnetizing magnetic metal vapor through the oblique evaporation process. The nonmagnetic underlayer film 16 is formed with a columnar area 16A, which contains a plurality of microscopic Co crystalline particles (hereinafter referred to as Co particles) and anther area 17A, which contains a plurality of microscopic CoO crystalline particles (hereinafter referred to as CoO particles), in the longitudinal direction of the base film (not shown). The columnar area 16A is formed in a shape that is perpendicular to the surface of the underlayer film 16 or slanted a little from the vertical direction of the underlayer film 16. A reference sign 18 is a magnetic film that is formed on the nonmagnetic underlayer film 16 by optimizing an injection amount of oxygen gas through the oblique evaporation process.
In other words, as mentioned above, the nonmagnetic underlayer film 16 of the thin film magnetic tape, which is produced by the conventional film-forming apparatus 20 through the oblique evaporation process, is composed of Co particles and CoO particles. Relative quantities of the Co particles and the CoO particles, which constitute each area of the nonmagnetic underlayer 16, are such that more CoO particles exist in the area 17A, which is allocated between columnar areas, and more Co particles exist inside the columnar area 16A.
The columnar area 16A having more Co particles is composed of a plurality of columnar areas 16a through 16n in the longitudinal direction. On the other hand, the area 17A having more CoO particles is composed of a plurality of areas 17a through 17n, which are sandwiched among the plurality of columnar areas 16a through 16n, in the longitudinal direction.
Consequently, a grain boundary, which fixes a boundary between the columnar area 16A having more Co particles and the area 17A having more CoO particles, is made clear.
Further, the magnetic film 18 is formed with columnar areas 18a through 18n having more Co particles and areas 19a through 19n having more CoO particles, which are sandwiched among the columnar areas 18a through 18n. 
The plurality of columnar areas 18a through 18n is in a shape of slanting with respect to the surface of the magnetic film 18. In a case that a thin film magnetic tape is produced by forming a CoO magnetic film 18 on the nonmagnetic underlayer film 16 having the columnar areas 16a through 16n by the oblique evaporation process while injecting oxygen gas, the concentration of oxygen gas is lower than the case of forming a nonmagnetic CoO film.
Collision probability of a Co evaporated particle with injected oxygen gas becomes lower than the case of forming the nonmagnetic underlayer film (CoO film) 16, so that the Co evaporated particle is relatively high in a rate of traveling in a straight line, and further the Co evaporated particle is incident into a surface of substrate with slanted because the initial incidence angle is set to “θ max” as shown in FIG. 15. Consequently, it is supposed that an aggregation of particles grows towards a slanted direction close to an incidence direction and forms a columnar construction.
A magnetic flux induction-type head has been used for a conventional recording and reproducing head. However, a carrier to noise (CN) ratio has been limited by the thermal noise of the magnetic flux induction-type head.
Further, a reproduction output decreases in relation to lowering a relative speed between a head and a tape due to a trend of miniaturizing a recording apparatus, and resulted in becoming hard to record and reproduce a signal in a high bit rate.
On the other hand, it is essential for advancing a recording medium towards higher density to thin a film thickness of a magnetic film as well as increasing coercive force.
In a case of coping with the trend for increasing coercive force higher by using the magnetic flux induction-type head, if a film thickness of a magnetic film is drastically thinned, a problem such that a reproduction output decreases and resulted in decreasing a signal to noise (SN) ratio occurs. In order to solve such a problem, an MR (magnetoresistive) head, which applies a magnetoresistive effect, draws attention. The MR head can obtain a high output, independent of a speed relative to a recording medium.
Further, the MR head has a low resistance value in a whole range of band, so that the MR head is advantageous to reduce drastically a thermal noise in comparison with a magnetic flux induction-type head.
An MR element of the MR head is, however, saturated and an operating range of the MR element exceeds a linear area when an amount of magnetic flux from a recording medium exceeds a specific amount. Consequently, the MR head generates distortion in waveform and asymmetry of pulse.
On the other hand, an amount of magnetic flux that is absorbed by the MR element is in proportion to Brδ, that is, a cross product of residual flux density Br and a magnetic film thickness δ. Therefore, it is essential to assign Br and δ that is most suitable for a MR head. Generally, a magnetic film thickness δ that is most suitable for a MR head becomes thinner than that of a magnetic flux induction-type head.
In a case that a film thickness is thinned as thin as less than 1000 Å so as to assign a magnetic film, which is formed by the oblique evaporation process, to the most suitable film thickness for a MR head, there arose a problem such that coercive force Hc decreases drastically.
With respect to a method of solving the problem, a method such that forming an underlayer film of nonmagnetic CoO, which is formed by evaporating Co as a magnetic metal material in an atmosphere of oxygen gas through the oblique evaporation process, on a nonmagnetic substrate and forming a CoO magnetic film on the underlayer film through the oblique evaporation process prevents coercive force Hc in the range of less than 1000 Å of magnetic film thickness from deteriorating has been suggested in the Japanese Patent No. 2988188 and in the publication: Japanese Society of Applied Magnetics”, vol. 121, No. 4-2 (1997), entitled “Effect of a CoO underlayer on CoO films evaporated obliquely”.
According to the method mentioned above, when forming a Co—CoO magnetic film on growing particles of an isolated CoO nonmagnetic underlayer film, by isolating growing particles of the Co—CoO magnetic film with following the growing particles of the CoO nonmagnetic underlayer film, magnetic interaction among particles of the Co—CoO magnetic film can be reduced. Consequently, the method prevents magnetostaitc characteristics from deteriorating even in an extremely thin Co—CoO magnetic film. However, there existed some problems mentioned below by this method.
By this method,    {circle around (1)} It is essential for the CoO nonmagnetic underlayer film to be formed in a thickness of more than 300 Å. Consequently, material cost increases.    {circle around (2)} In a case of forming the CoO nonmagnetic underlayer film and the CoO magnetic film, a base film 6 must be passed through the tape-running path shown in FIG. 15 twice. Consequently, mass-productivity is extremely deteriorated.    {circle around (3)} Due to the 2-time running of the base film 6 as mentioned above, the provability of attaching dust on and scratching the surfaces of the substrata and the underlayer film increases, and resulted in increasing dropout of a thin film magnetic tape that is completed.    {circle around (4)} In order to solve the problem of inferior mass-productivity and complete the production of thin film magnetic tape by running the base film once, it is essential for the cooling can roll 7, the piercing electron gun 12 and the crucible 8 that two set of them are installed respectively. Consequently, too much facility introducing cost is not practical.
In consideration of the above-mentioned problems, it is found that the problems occur in an area such as a material of nonmagnetic underlayer film and its film thickness.
Further, it is also found that desired magnetic characteristics can not be obtained in some cases due to conditions of fine particles, which constitute a nonmagnetic underlayer film, although the nonmagnetic underlayer film has been formed excellently.